ENTHALPY CHANGES AND HEAT-CAPACITY CHANGES IN TRANSFORMATIONS FROM HIGH-SURFACE-AREA AMORPHOUS ICE TO STABLE HEXAGONAL ICE

Amorphous ice prepared by slow condensation of water vapor onto a surface cooled with liquid nitrogen or liquid hydrogen undergoes several exothermal transformations on warming. Thermal analysis by means of warming curves indicates that the heat liberated on warming ice prepared at -253°C is 8 cal/g from -253° to -196°C and 22 cal/g from -196° to -120°C. Ice prepared at -196°C gives up 14 cal/g between -185° and -125°C. These enthalpy changes are attributed in part to decreases in the surface area of the amorphous ice. Beginning at -120°C, 24 cal/g heat is evolved on crystallization to the cubic form (ice Ic). Between -80° and -50° an additional 3 cal/g is attributed to recrystallization of the cubic phase to larger crystals. In the cubic to hexagonal (ice Ih) transition between -50° and -5°, ΔH is less than ±0.3 cal/g, but a decrease occurs in the low-temperature heat capacity. The heat capacity of amorphous ice or ice Ic in the range -170° to -120°C exceeds that of ice Ih by 27% and the heat capacity of ice Ic between -120° and - 70°C exceeds that of ice Ih by 12%. The higher heat capacity of ice Ic is attributed to a greater degree of order at low temperature. Heat evolved in the hypothetical transition from amorphous ice at -253°C to ice Ic at the same temperature is 55±5 cal/g. No glass transition was observed in amprophous ice. Within experimental error, the enthalpy and heat-capacity changes are the same in D2O as in H2O.